Not qualified to give tax advice, but I'll tell you what worked for me!
I was a Special/Reserve PO for 18 years. Other than some "hand me downs" from FT POs that voluntarily turned in clothes that they had outgrown, we had to buy all our own gear, with NO allowances from PD. We also drove our own cars around on Halloween (and at numerous road races each year) with blue lights temporarily mounted on the roof as the PD didn't have spare vehicles and 35 of us were a larger force than the FT department.
Any special clothing that can't be considered "street clothes" can be legally deducted, along with mileage (working Halloween, road races with POVs blocking streets, etc.), equipment, uniform cleaning expenses, association memberships, training expenses, etc. [Detectives wouldn't be able to deduct the cost of suits, ties, white shirts since they are "street clothes". Uniformed officers will be able to deduct all such costs.]
I was audited by the IRS for business expenses twice and presented my records and they didn't blink! I won my case both times at the audit regarding any LE related business expenses. I found that the key was keeping meticulous records.
Go to the IRS website and DL the Form 2106 (Employee Business Expense) and the booklet associated with it (gives lots of good info) and you'll be good to go.